Prevention of corrosion in process of extracting mercaptans



Patented Oct. 23, 1951 ATENT OFFICE PREVENTION OF CORROSION IN PROCESS OF EXTRACTING MERCAPTAN S Ronald A. Baker, Los Angeles, Calif., assignor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y. a corporation of New York No Drawing. Continuation of application Serial No. 541,657, June 22, 1944. This application May 27, 1947, Serial No. 750,896

1 Claim. 1

The present application is a continuation of my copending application Serial No. 541,657, filed June 22, 1944, now abandoned, and entitled Protection of Metals from Caustic Sulfur Corrosion.

This invention relates to means for protecting from corrosion and embrittlement the apparatus in which steel surfaces are contacted at relatively high temperature with strong aqueous solutions of caustic alkalis containing organic acids and dissolved sulfur compounds.

A well-known example of a process of this type is the so-called caustic cresylate treatment of asoline for the removal of mercaptans. In this process the mercaptan-containing gasoline is contacted, at about atmospheric temperature, with an aqueous liquid containing, for example, 300 grams per liter of sodium hydroxide and a slightly smaller proportion of cresylic acid. Contacting being produced by counterfiow, in packed towers or other contacting vessels, the mercaptans contained in the gasoline pass into solution in the aqueous phase and with it are separated from the gasoline.

In this low temperature treatment, corrosion of the apparatus is not a problem, but both corrosion and embrittlement become serious in the ensuing stage of regeneration. In this stage the aqueous caustic solution is heated, as say from 240 to 260 Fahr., in interchangers, and is then blown with steam as in bubble towers, the mercaptans being thus volatilized and recovered in the condensate While the treating solution is brought back to its original strength and activity.

In modifications of this process, potassium hydroxide is substituted for sodium hydroxide, and tannic acid or other organic acid is substituted for cresylic acid. In all of these modifications regeneration is effected in substantially the same manner and in all of them corrosion is responsible for the rapid destruction of the regenerating equipment. The cause of this abnormal rate of corrosion is not fully determined, but in some manner the solvent tendency of strong alkali solution toward iron is enhanced by the dissolved mercaptans, and in practice corrosion is a serious burden on these processes, which has not heretofore been overcome.

I have discovered that by the addition to the treating solution of a relatively small quantity, for example from 0.1% to 1.0% by weight, of an alkali-metal phosphate, corrosion and embrittlement are substantially prevented and the life of the regenerating equipment is thus greatly extended. The metallic surface in contact with the solution, which normally remains bright, rapidly accumulates a coating which is believed to consist of an iron phosphate or phosphates and which is highly resistant to attack by alkalis. This coating is strongly adherent to the metal, but if for any reason a portion of it becomes detached, the coating is immediately reproduced and thus the protection is continuously maintained.

The content of alkali-metal phosphate is consumed very slowly and needs to 'be made up at long intervals. The presence of the phosphate has no effect on the mercaptan-removing process, which in every manner functions exactly as though the salt were absent.

Apparently the enectlveness of the added phosphate is due to the dissociated P04 ion and thus the various alkali-metal phosphates are effective in proportion to their phosphorus content. Any of the alkali-metal phosphates, such as the hexanleta-, (11-, tri-, or tetra-salts with sodium or potassium may be used, though by reason of its low cost and relative high phosphorus content I prefer to use trisodium phosphate for this purpose.

While the method above described has been tested most thoroughly in the regeneration of caustic-cresylate solutions, it is equally effective in inhibiting corrosion in the regeneration of caustic-tannin solutions and others in which a strong aqueous solution of a caustic alkali in combination with an organic acid is used as a solvent for mercaptans and regenerated by heatmg.

I claim as my invention:

In a process of regenerating a strong aqueous solution of caustic alkali containing dissolved petroleum mercaptans, which process includes application of heat in the presence of steel surfaces to volatilize said mercaptans, the method of inhibiting corrosion of said steel surfaces which comprises, maintaining in said solution a small quantity, from 0.1% to 1.0% by weight, of alkali-metal phosphate.

RONALD A. BAKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

